During orthopedic surgery, it is often the case that surgeons are required to make surgical alterations to bone. Such alterations include but are not limited to making cuts in bone, drilling holes in bone, and affixing a plate, screw, nail, or prosthesis to bone.
When making such alterations, it is desirable that the alteration be realized in a manner which precisely conforms to the operative plan of the surgeon. Among the aspects of a surgical alteration which require careful control are: (1) the alignment of the cut, hole, plate, screw, nail, or prosthesis with respect to the anatomy of the patient; (2) if there is more than one bone cut and/or hole, the relative alignment of the cuts and holes with respect to each other; and (3) if a plate, screw, nail, or prosthesis is to be fixed to bone, the alignment of the cuts and/or holes with respect to the plate, screw, nail or prosthesis.
Current surgical techniques utilize limited mechanical means to assist the surgeon in making bone alterations. However, existing techniques do not suffice to ensure that perfect or nearly perfect alterations can be achieved routinely. Where practical, it is desirable to enhance the surgeon's decision-making process by providing accurate solutions to purely geometric problems posed by surgery, while leaving final positioning decisions up to the surgeon. When the surgeon is provided with accurate geometric solutions, the quality of the overall subjective evaluation should be improved.
An example of a surgical procedure that requires accurate geometric solutions, as well as the evaluation of specific patient physiological characteristics, is total knee arthroplasty (TKA), which is a total knee reconstruction surgery. The anatomic knee is a remarkable mechanism. Contrary to first impression, it is not a simple hinge. Rather, the femur and tibia move relative to each other with a complex mixture of rolling and sliding motions. The stability of the joint comes entirely from soft tissue structures, not from bone geometry. The major stabilizing ligaments are the medial and lateral collateral ligaments, and the anterior and posterior cruciate ligaments.
In total knee arthroplasty, the distal femur and the proximal tibia are resected and are replaced by prosthetic components made of metal and plastic. The most successful designs are unconstrained prostheses that closely mimic the natural anatomy of the knee. Like the anatomic knee, unconstrained designs allow the femur and tibia to roll and slide relative to each other. They depend on the natural ligamentous structures of the knee to stabilize the reconstructed joint.
Total knee reconstruction surgery is conceptually simple. The knee is flexed, the patella moved to one side to give access to the joint, and the degenerated surfaces of the femur and tibia are cut away. The bone cuts are made to fit femoral and tibial prosthetic components, which are available in a wide variety of sizes and styles. These are generally cemented into place, using polymethyl methacrylate (PMMA). One new technique uses no cement. Rather, bone grows into a porous backing on the prosthetic component. This is termed porous-ingrowth fixation.
Each year, approximately 100,000 people undergo a TKA. TKAs are often performed in people whose knees have become so painful, because of progressive arthritic changes, that they are unable to rise from a chair, walk, or climb stairs. For these people, total knee arthroplasty can provide a return to near-normal, pain-free life.
A great deal of developmental technology has gone into perfecting the femur prostheses used in TKAs. However, the technology for positioning the prosthesis properly on the femur has not similarly advanced. Ideally, the bone cuts should be (1) an exact press-fit to the components, and (2) in proper alignment with respect to bones and soft tissues. Failure to achieve these goals will result in poor knee mechanics and/or loosening of the components, leading eventually to failure of the reconstruction.
At present, the surgical instrumentation used in total knee arthroplasty consists of hand-held saws which are guided by simple cutting blocks and mechanical jig systems. There is abundant evidence in the literature that these tools do not suffice to do a good job. First, most prosthetic components are not put in with perfect alignment, and misalignment of three to five degrees or more is not uncommon. Second, prosthetic components do not fit perfectly on the bone, and there are inadvertent gaps between the cut surface of the bone and the prosthesis. Third, there is a learning curve associated with arthroplasty technique. The first fifty knees a surgeon does are not as good as subsequent knees.
The primary goals of the surgeon during total knee arthroplasty are: proper alignment of the reconstructed knee, stability of the reconstructed knee, and press-fit of the components to the bone. With respect to alignment, the knee should neither be knock-kneed or bowlegged, to ensure that the medial and lateral sides of the components bear equal loads. Asymmetric loading leads to early failure. In addition, the ligaments of the knee should provide stability at all angles of flexion, as they do in the anatomic knee. If the ligaments are too tight, they will restrict the motion of the knee. If they are too loose, the knee will "give way" during use.
Finally, if a prosthetic component is even slightly loose, then each step will "rock" the component against the bone. The bone soon gives way, and the reconstruction fails. Ideally, the prosthesis is a press-fit to the cut bone at the time of surgery. This minimizes micro-scale rocking motions. Press-fit is especially important for a porous ingrowth prosthesis, since even a one-millimeter gap between prosthesis and bone is too large for the ingrowth process to bridge.
These goals are simple to state, but difficult to achieve in the operating room. To understand the problems, consideration should be given to all the ways malalignment can occur. There are three different ways a component can be malaligned in orientation. These correspond to rotations of the component away from the desired orientation along the internal/external, varus/valgus, and flexion/extension axes. Similarly, there are three different ways to malposition a component by translation along an axis. These correspond to translations along the medial/lateral, proximal/distal, and anterior/posterior axes.
Thus, to achieve good alignment and good ligament balance, surgeons must mentally manipulate three translational and three orientational variables for each of the femoral and tibial components, or twelve spatial variables in all. Margins for error are small. Repositioning the prosthetic component by even one millimeter has an appreciable effect on the stability of the knee. Moreover, each knee presents its own special problems. It is frequently the case that the knee has a preexisting deformity which must be taken into account.
In addition, the surgeon must also take care that the bone surfaces are press-fit to the component. This involves five cut planes and two drill holes for a typical femoral component, and one cut plane and two drill holes for a typical tibial component, for a total of ten separate cutting operations. In each case, imprecision of one millimeter or less can have significant consequences, especially for porous-ingrowth prostheses.
It is a remarkable fact that present-day surgical instruments for total knee arthroplasty could have been manufactured in the nineteenth century. The essential features of present-day instrumentation systems are their reliance on hand-held oscillating saws to make bone cuts, and mechanical jigs with slots and cutting blocks to help align the cuts. Considerable ingenuity has been applied to optimizing instrumentation systems of this type, and there are dozens of variations on the market. Nonetheless, poor alignment and inaccurate cuts are common problems when using these mechanical instrumentation systems.
Poor alignment occurs when femoral and tibial cutting jigs are not properly aligned with respect to the hip, the ankle, and the stabilizing soft tissues of the knee. This can happen because the surgeon is mislead by the anatomic landmarks used by a given system, because the landmarks are concealed by fat and muscle, because preoperative deformities exist, or because the jig has shifted slightly during the procedure. The best test of alignment is flexion of the newly reconstructed knee. Unfortunately, by the time such a test can be made, the bone cuts have been made, and it is too late to change the alignment of the components.
Inaccurate cuts occur when the various cuts and drill holes do not precisely mate with the surfaces of the prosthetic components, possibly as the result of errors which accumulate during placement and removal of the various cutting blocks. Also, there is inherent inaccuracy associated with a flexible, oscillating saw blade resting on a cutting block or in a slot. The blade tends to "sky" when it encounters a dense section of bone. This tendency is resisted by canting the handheld saw in a downward direction.
There is ample evidence in the published literature that the present state of total knee arthroplasty is not satisfactory. Cameron H. U., Hunter G. A. in: Failure in Total Knee Arthroplasty, 170 Clinical Orthopaedics and Related Research: pp. 141, 146, 1982, noted, "[t]he results of total knee arthroplasty range from an acceptable 5.4% failure rate at five years to an abysmal 70% failure rate at three years. Failure rates of this magnitude indicate that many revisions are being performed." Bryan R. S., Rand M. J. in: Revision Total Knee Arthroplasty, 170 Clinical Orthopaedics and Related Research: pp. 116-122, 1982, state that, "[p]roper component alignment is of critical importance" and that "[f]ailure to obtain appropriate component orientation, axial alignment, and soft tissue balance predisposes implants to loosening and failure." Hood R. W., Vannie M., and Install J. N., as noted in, The Correction of Knee Alignment in 255 Consecutive Total Condylar Knee Replacements, 160 Clinical Orthopaedics and Related Research: pp. 94-105, 1981, found in a series of 255 knees that, "[e]leven percent of the knees in this series were outside the alignment limits selected. This may reflect extremes of body habitus but, more importantly, indicates that deficiencies in instrumentation still remain." Hvid I., Nielsen S. in: Total Condylar Knee Arthroplasty, 55 Acta Orthop Scand 55: pp. 160-165, 1984, found in a study of 138 knees that although "the aim was to place the tibial component at right angles to the tibial axis," only "53 percent were within four degrees of tilt in any direction." Some of their components were eight degrees or more out of alignment. In summary, there is ample evidence that with existing instrumentation surgeons cannot obtain good alignment routinely in total knee arthroplasty.
As is evident from the less-than-satisfactory clinical results, the theory and practice of jig-assisted knee surgery are two different things. In practice, total knee arthroplasty is largely a seat-of-the-pants procedure. Surgeons recruit every pair of eyes in the operating room to judge how a contemplated cut "looks" from a variety of angles. Equally important is a steady and practiced hand on the cutting saw, and a sound understanding of the biomechanics of the knee joint.
The conventional TKA requires that the surgeon attempt to achieve exact physiologically correct relationships and to make geometrically exact cuts with inexact methods. As discussed above, both the position and quality of the cuts and bores greatly affect the success of the operation. While the background of a TKA has been described, numerous types of surgeries present the same problem of integrating geometric analysis with a subjective evaluation of physiological factors. Examples of such surgeries are osteotomies and ligament repairs. In the majority of these operations, certain mechanical devices, such as the jig systems described above, have been developed to aid in the operation. The exactness of these mechanical devices varies and, thus, so do the efficiencies resulting from their use. However, most surgical procedures that are not solely based on subjective medical decisions will suffer from some inaccuracies based on the fact that surgeons have a limited capacity for making independent exact geometric calculations and carrying out tasks based on those calculations.